ABS

The guard for abs requires its argument to be a rational (real,
in ACL2(r)) number.

Abs is a Common Lisp function. See any Common Lisp documentation
for more information.

From ``Common Lisp the Language'' page 205, we must not allow
complex x as an argument to abs in ACL2, because if we did we
would have to return a number that might be a floating point number
and hence not an ACL2 object.